U.S. Pat. No. 4 321 234, the entire contents of which are incorporated herein by reference, discloses a reactor wherein the heat of reaction is removed by coolant that circulates through a number of vertically extending cooling tubes which are arranged parallel to the axis of a single, cylindrical catalyst bed of the reactor. A feed gas flows in the radial direction through the catalyst bed and thereby undergoes an exothermic reaction. The heat of reaction causes the coolant within the cooling tubes to boil. The coolant is fed through the cooling tubes under pressure, at its boiling temperature. After absorbing the heat of reaction, the coolant is discharged from the reactor in the form of a vapor, or a liquid-vapor mixture of the coolant. In the specification, the vapor and the mixture of vapor and liquid are, in combination, called merely as a vapor of the coolant.
U.S. patent application Ser. No. 502,298, now U.S. Pat. No. 4,594,227 filed Sept. 8, 1983, the entire contents of which are incorporated herein by reference, which corresponds to Japanese patent application No. 167639/1982, discloses a reactor wherein the annular, cylindrical catalyst bed formed in the intercylinder space between a pair of inner and outer cylindrical catalyst retainers is subdivided by two or more partition walls which extend radially and vertically. By this means, a plurality of catalyst beds which are sectorial in horizontal cross section are formed. The reactant feed gas is flowed radially through the different catalyst beds in a predetermined sequence, while a coolant liquid is fed through cooling tubes installed in each of the catalyst beds in the same manner as disclosed in U.S. Pat. No. 4 321 234. Using the improved arrangement of U.S. Ser. No. 530 298, the temperature distribution in the direction of the gas flow in the catalyst beds can be optimized in accordance with the purpose and characteristics of the particular reaction that is to be performed. In addition, the heat of reaction can be recovered by making use of the coolant vapor. The reactor of Ser. No. 530 298 is accordingly advantageous for effecting exothermic reactions.
The present inventors have discovered that the outer pressure vessels of the foregoing reactors, according to the prior art, suffer from hydrogen embrittlement. Hydrogen embrittlement occurs when the outer pressure vessel is exposed to a high temperature, high pressure, feed gas containing a large amount of hydrogen. The present invention relates to a reactor which retains the advantageous features of the foregoing two prior art reactors, and which, in addition, is provided with unique structural features that render it less subject to hydrogen embrittlement.
Hydrogen embrittlement can occur when a high temperature, high pressure, feed gas containing a large amount of hydrogen and/or a product gas which also contains a substantial amount of residual hydrogen are brought into contact with a conventional low carbon steel or alloy steel containing a total of 10% by weight or less of alloying components other than iron and carbon. The foregoing product gas is formed from the feed gas by bringing the feed gas into contact with the catalyst in the catalyst bed. After a long period of such contact, the well-known phenomenon called hydrogen embrittlement takes place whereby the low carbon steel or alloy steel is deteriorated by the action of hydrogen and becomes brittle.
As used hereinafter, the term "carbon steel" refers to steel consisting essentially of 0.02 to 0.6 wt. %, preferably 0.1 to 0.4 wt. % carbon, less than 10.0 wt. % preferably less than 5.0 wt. % alloying elements other than iron and carbon, and the balance is essentially iron. The term "carbon steel" used hereinafter refers to both low carbon steels and alloy steels containing up to 10 wt. % of alloying components other than iron and carbon.
As a method for minimizing such hydrogen embrittlement, as disclosed in U.S. Ser. No. 530,298 noted above and many other literatures, a low temperature feed gas can be fed to a reactor having an outer pressure vessel made of carbon steel. Before this low temperature feed gas reaches the reaction temperature, it is first flowed along the inner surface of the outer pressure vessel in order to reduce heat transfer from the catalyst bed of the reactor, which is at a high temperature, to the outer pressure vessel, thereby maintaining the temperature of the outer pressure vessel at 300.degree. C. or lower, preferably 250.degree. C. or lower. Alternatively, an outer pressure vessel can be used which is made of a stainless steel containing more than 10 wt. % of alloying elements other than iron and carbon.
According to methods wherein a low temperature feed gas is flowed along the inner surface of the outer pressure vessel made of carbon steel, the low temperature feed gas can be preheated by causing it to undergo heat exchange with high temperature product gas leaving the catalyst bed, using a heat exchanger that is installed in the reactor. However, the amount of heat needed to preheat the feed gas is less than, the sum of the evolved heat of reaction and the heat content of the product gas leaving the catalyst bed. Consequently, the product gas flowing out of the reactor is normally high in temperature in spite of the heat exchange step, and thus it is not possible to use the product gas for the purpose of preventing the outer pressure vessel made of carbon steel from increasing in temperature by flowing the product gas along the inner surface of the outer pressure vessel after the heat exchange step. The foregoing prior art procedures fail to eliminate the need to make the outer pressure vessel from stainless steel in order to avoid hydrogen embrittlement. The use of stainless steel for making the outer pressure vessel, however, renders the reactor more expensive.
When a low temperature feed gas is first flowed along the inner surface of the outer pressure vessel as described above, the pressure in the flow passage adjacent to the inner surface of the outer pressure vessel is higher than the pressure in the central part of the reactor containing the catalyst beds by an amount corresponding to the pressure drop that occurs due to the flow resistance that the gas encounters as it passes through the catalyst bed and the other flow passages in the reactor. This pressure drop is normally on the order of 5 to 20 kg/cm.sup.2. The feed gas flowing along the inner surface of the outer pressure vessel is upstream of the gas that is passing through the catalyst bed in the central part of the reactor. Under these circumstances, an external pressure corresponding in magnitude to the foregoing pressure drop is exerted on the external cylindrical partition wall which separates the chambers that differ in pressure. This external pressure tends to collapse the cylindrical partition wall or walls inwardly, with the practical effect being that the cylindrical partition wall will be deformed by buckling. In order to prevent such buckling, the cylindrical partition wall(s) and the radial partition wall(s) which aid in supporting the cylindrical partition wall(s) must be made thicker.
It has also been generally necessary to fabricate the interior cylindrical and radial partition walls mentioned above from stainless steel, because these walls come into contact with gases containing a large amount of hydrogen at a high temperature and pressure. Thus, these partition walls must be made both thicker and of a more expensive material in order to prevent buckling and hydrogen embrittlement. These problems have been generally overlooked in the prior art because the prior art reactors have been built on a relatively small scale. However, it has recently become possible to greatly enlarge the reactor in which a high pressure feed gas containing a large amount of hydrogen is used at a high temperature to synthesize ammonia, methanol and similar products. When such a reactor is built on a large scale, it becomes important to solve the problems of hydrogen embrittlement and buckling of the interior partition walls without using large amounts of expensive stainless steel.